Exhaust gas sampling systems that utilize partial flow dilution tunnels (PFDT) have been used since the early 1990s as an effective alternative to full dilution tunnel systems for development and certification of engines where steady state model testing was permitted. Previously, all off-highway and until recently much of European bound on-highway engine testing and certification was performed with systems utilizing PFDT's, due to the fact that they are more portable, less expensive and often more repeatable than their full dilution counterparts. Regulatory organizations such as ISO, CARB, EPA and EEC all permit the use of PFDT's for steady state test cycle certification. One such system is disclosed in co-owned U.S. Pat. No. 5,058,440, issued on Oct. 22, 1991 to Russell R. Graze, the inventor of the presently claimed subject matter.
More recently, the environmental protection agency has declared its interest in promulgating transient cycle regulations of large off-highway diesel engines in order to better control emissions output from these engines. These regulations are expected to be in effect by 2006. The size of the off-highway diesel engines to be regulated eclipses the mass flow rate capacity of the industries full dilution tunnels that have been in use for the past twenty plus years to quantify on-highway engine emission levels, including particulate matter. Furthermore, the sheer number of off-highway ratings to be developed, in combination with concurrent regulatory pressure placed on-highway engine development times, nearly precludes the use of existing full dilution tunnels for off-highway development, even for small engines.
Therefore, it became desirable to develop a PFDT that could be used to test and certify off-highway diesel engines under transient conditions, and consequently would be also available to be utilized to test on-highway engines under transient conditions as well. One such system is described in co-owned U.S. Pat. No. 6,615,677, issued on Sep. 9, 2003 to Richard R. Dickson and Russell R. Graze. In this system, the mass flow of both air and fuel supplied to the engine are continuously monitored by a controller. When a change is sensed, the controller commands corresponding changes in the gas sampling system in order to accommodate for the inherent change in the exhaust mass flow rate. Although this system has proven effective in quickly responding to transient air and/or fueling supply changes to the engine, there remains room for improvement. For instance, if the controller commands a change to the sampling system too quickly after an air or fuel supply change has occurred, the exhaust may be over or under sampled for a brief duration, resulting in less than satisfactory data. The time lag between changes in the engine exhaust and those changes reaching the gas sampling probe relate to the fact that the probe is typically located downstream in the exhaust stack, and preferably downstream from any exhaust aftertreatment devices. Correctly adjusting for this time lag can be even more difficult if the sampling system is to be made more versatile and useful with a variety of different sized engines.
In the past, in order to perform transient emissions testing, the use of full flow dilution tunnels, or a CVS system, was required. These devices are large, expensive, difficult to maintain and are engine size specific. The co-owned U.S. Pat. No. 6,615,677 responded to this problem by demonstrating a partial flow dilution tunnel that was capable of achieving proportional sampling by rapidly changing dilution air flow to the dilution tunnel described in earlier patent U.S. Pat. No. 5,058,440. These systems operate well in situations where the engine displacement relative to the exhaust system volume does not change appreciably, since the response time of the partial flow sampling system is fixed. However, the current versions do not address applications where the engine size is very small relative to the volume of the exhaust system upstream from the partial flow system sample probe location, or in cases where exhaust after treatment research requires that a significant exhaust system volume be located between the engine and the partial flow system sample probe location. In relatively extreme cases where the ratio of the exhaust volumetric flow rate at standard temperature and pressure to the volume of the exhaust stack between the turbocharger outlet and the sample zone is less than approximately 550:1 for a 300 millisecond response time system, particulate emissions over-sampling can result during accelerations. This is due to the sample mass fraction of the exhaust flow advancing past proportionality. In response to this problem, a modification must be made to the partial flow sampling system to insure that proportional sampling consistently occurs.
The present disclosure is directed to overcoming one or more of the problems set forth above.